Today, many countries, especially China and Arab countries, are building artificial islands, but they are solid, not floating. This is usually done in places where the sea is very shallow. To make a solid island in a place where there are greater depths would require a lot of materials, energy and money.
At greater depths, it is only possible to create a floating island.
The best material for this could be plastic bottles. A well-sealed plastic can can withstand very high pressures and can last hundreds of years. Tightly sealed bottles should be placed in large plastic mesh bags that would be placed on top of the water. Such plastic bags should be well connected to each other when placed in the water. After placing the first row of bags with plastic bottles, the entire surface should be covered with a solid plastic net, and a second row of bags with plastic bottles should be placed on it. On this second layer, a strong plastic mesh should be placed again. Another row of bags with plastic bottles should be placed on top of that. In this way, by adding new rows, the floating island should be built up to the desired height. After placing the last row of bags with plastic bottles, construction expanding foam should be spread over the surface, which should fill the gaps between the upper row of plastic bottles. When the foam dries, another dense and solid plastic mesh should be placed over it, and a steel mesh over which concrete with a higher content of glass fibers should be poured. Fiber glass is supposed to protect the concrete from cracking in case of slight bending in larger waves and storms.
The width and length of such an island could be from several tens of meters to several hundreds of meters.
Floating restaurants, floating hotels, floating houses, docks for boats and ships, shops and the like could be placed on such an island. In this way, it would be possible to build smaller, narrow and long islands that could be brought to a specific place on rivers and placed across the river as a pontoon. If you want to build a floating airport on it, the island should be up to 100 meters wide and several kilometers long. The island would sail with the help of smaller strong ships that would pull or push it. It would withstand stormy wind blows, and it could only be damaged by long transoceanic waves that can be several tens of meters high with a distance between waves of one hundred to two hundred meters.
Some of my other innovations and technical analysis.